Our emphasis is shifting to study of expression of cloned genes reintroduced into living cells in tissue culture or intact animals. Expression of HSV-tk and DHFR genes transferred into intact animals has been obtained for the first time and globin genes are under intensive investigation. In addition we are using cloned mRNA genes from a human leukemic cell line (HL60) as probes to study regulation of the large groups of genes turned off or on during differentiation of neutrophils or granulocytes. This work is facilitated by the fact that we can induce differentiation of these cells in vitro.